Silicon micromachining has been developed over the last decade as a means for accurately fabricating small structures without requiring assembly of discrete components. Such processing generally involves selective etching of a silicon substrate and depositions of thin-film layers of semiconductor materials. Recently, silicon micromachining has been applied to the fabrication of structures that include a rotary or linear bearing, which bearings allow substantially unrestricted motion of a moving component in one degree of freedom. Such bearings have enabled the development of electrically-driven motors, referred to herein as micromotors. Such micromotors have a planar geometry, with gap separations on the order of 1-2 microns and lateral dimensions on the order of 100 microns or more.
The small sizes of such micromotors and the characteristics of silicon micromachining combine to produce electro-mechanical characteristics which significantly differ from those of conventional motors. Electrostatic forces are generally found to be larger than the magnetic alternatives for devices scaled to micro-dimensions. The use of micro-size field-generating structures, enables increased strength electromagnetic fields to be created.
Recently, Tai et al. have reported the fabrication of a micromotor including a rotor pinned to a substrate or stator by a central bearing that restricts its lateral and axial motion. See "IC-Processed Micro-Motors: Design, Technology and Testing", IEEE Solid State Sensor and Actuator Workshop, Hilton Head Island, South Carolina, June 4-7, IEEE Catalogue No. 90CH2783-9 (1990), pages 1-6. The entire structure shown by Tai et al. was micromachined from silicon using deposition and etching steps.
The manner of energization of the Tai et al rotor is via a variable-capacitance, side-drive arrangement, wherein stator poles are arranged about the periphery of the rotor. By appropriate energization of the side-disposed stator poles using a multi-phase signal, rotation of the rotor is achieved. Further details of this structure can be found in the following two papers by Mehregany et al., "Friction and Wear in Microfabricated Harmonic Side-Drive Motors", IEEE Solid State Sensor and Actuator Workshop, Hilton Head Island, South Carolina, June 4-7, IEEE Catalogue No. 90CH2783-9, pages 17-22 and "Principles in Design and Micro Fabrication of Variable Capacitance Side-Drive Motors", Journal of Vacuum Science Technology, A8(4), July/August 1990, pages 3414-3424.
The side-drive micromotor is the simplest electrostatic micromotor. Torque is derived via position-dependent capacitance between the rotor and stator poles. However, because of the side-by-side arrangement of the rotor and stator poles, field coupling is less than optimal and, as a result, the torque characteristics of the motor suffer. Furthermore, the rotor still experiences significant frictional engagement between itself and the central bearing that pins it to the substrate.
It is an object of this invention to provide an improved micromotor that exhibits lessened frictional losses.
It is another object of this invention to provide an improved micromotor exhibiting increased rotor-torque characteristics.
It is another object of this invention to provide an improved micromotor exhibiting a substantially linear-rotor torque characteristic.